Flow brakes or retarders have been used in vehicle drive systems as sustained action brakes or as service brakes in combination with friction brakes. Such uses of flow brakes have been generally confined to either of two drive system configurations. In one configuration, the flow brake was engaged by the output shaft of a speed change gear, while in the other, the brake was engaged by the vehicle engine. When the flow brake was engaged by the vehicle engine it was referred to as a primary retarder.
Many disadvantages were encountered with the use of flow brakes in either configuration. For example, where the flow brake was engaged by the output shaft of the speed change gear, large flow brake dimensions were required in order to provide adequate braking effect over a large input speed range, i.e. vehicle speed range. This resulted in power dissipation problems because no load losses were incurred even after the fluid medium had been evacuated. Large flow brakes generated relatively high no load torques through air flow resistance. Additional components were required to reduce air flow losses. Such components included diaphragms to preclude air ventilation.
With the flow brake in the primary retarder configuration, the brake was engaged by the engine output shaft and, in each speed of the speed change gear, its braking power increased sharply with rotational speed and reached the same value in each gear speed at the rated speed of the engine. This effect was substantially the same as that of a motor brake.
Further, the primary retarders did not usually include mechanisms for avoiding no load losses when disengaged, such as ventilation blocking diaphragms. Thus, if they were dimensioned for sufficient braking moments, significant no load losses occurred in all speeds. Additionally, retarders were not infinitely variable.
In the other configuration, with the flow brake at the output end of the speed change gear, the braking moments differed greatly in the individual shift speeds, because of the different gear ratios, and high braking moments were attained only in the lower shift speeds.